| As the increase of the proportion of clean energy in China’s energy structure and new energy vehicles on the power battery performance requirements,the current energy storage system has been unable to meet the needs of China’s economic and social development Therefore,the development of high-performance electrochemical energy storage devices is of great significance to realize green,low-carbon and high-quality development.The high energy density,low cost and relative safety of metal-air batteries make them more competitive in the next generation of energy storage devices.Among them,lithium-oxygen(Li-O2)batteries and zinc-air(Zn-air)batteries are the two most researched metal-air batteries at present,showing great potential for application.However,the electrochemical performance of metal-air batteries is still unsatisfactory,with problems such as low real energy density,large cell polarization and poor cycling stability.These problems mainly stem from the sluggish oxygen reduction and oxygen evolution reaction kinetics of metal-air batteries as well as the parasitic reactions induced by various components of the batteries.Through the design of catalyst microstructures,it is possible to optimize the adsorption energy of the catalyst to the reaction intermediates,modulate the reaction mechanism,and lower the electrochemical reaction energy barrier,thus obtaining metal-air batteries with higher energy density and stability.Metal-organic frameworks(MOFs)have many advantages,such as high porosity,welldefined catalytic center,diverse and adjustable components,etc.MOFs-derived materials can not only inherit the advantages of MOFs,but also obtain bifunctional catalysts by modulating MOFs precursors.Therefore,MOFs and their derivatives show a broad application prospect in the field of electrocatalysis.In this paper,we designed the morphology and optimize the electronic structure of MOFs and their derivatives by combining various strategies,such as surface-interface engineering,heterostructure construction,and defect modulation,to modulate the adsorption energy of the catalyst surface and reaction intermediates,as well as to analyze the conformational relationship between the catalyst microstructure,reaction mechanism,and electrochemical performance.The main research results show that:1.Nitrogen-doped carbon nanosheets loaded with tungsten-doped Co2P(W-Co2P/NC)were synthesized by salt-assisted and in situ phosphatizing combination technique.To address the low active site density of Co2P,high valence 5d transition metal tungsten doping was introduced to modulate the electronic structure of Co2P and enhance the number of active sites.In addition,the combination of salt templates and MOFs derivatization methods can effectively inhibit the agglomeration of Co2P nanoparticles,increase the ratio of exposed atoms on the surface and enhance the catalytic activity.The MOFs-derived nitrogen-doped carbon nanosheets have high electrical conductivity and good ORR catalytic performance,providing fast charge transfer to the anchored Co2P.The tungsten atoms have a strong electron-donating ability to enhance the electron density of the surrounding atoms,which optimizes the adsorption energy of Co2P for ORR/OER intermediates and reduces the reaction energy barrier.The catalyst exhibited high ORR(half-wave potential of 0.861 V)and OER activity(overpotential of 350 mV at the current density of 50 mA cm-2).The Zn-air battery assembled with WCo2P/NC cathode exhibited high power density(224.4 mW cm-2),superior specific discharge capacity(881 mAh g-1)and long cycle life(130 h).2.ZIF-67-derived hollow NiCo LDH/MnO2 nanocages were prepared by in situ etching and hydrothermal reaction.The introduction of Ni2+induced the hydrolysis of ZIF-67,which was transformed to produce hollow nanocages assembled by NiCo LDH nanosheets.After hydrothermal reaction,δ-MnO2 was grown on the outside of the nanocage to construct the NiCo LDH/MnO2 heterostructure.The hollow nanocage structure effectively inhibits the stacking of NiCo LDH and MnO2 nanosheets,provides larger specific surface area and more exposed active sites,shortens the mass-transfer channels.The introduction of MnO2 not only enhances the stability of the NiCo LDH structure,but also accelerates the electron transfer,optimizes LiO2 adsorption energy and controls the morphology of discharged products.Under the dual effects of LiO2 adsorption energy and catalyst surface morphology,the Li2O2 grown on the NiCo LDH/MnO2 cathode is a toroid particle assembled by interlaced nanosheets.Li2O2 has abundant surface structure and tightly contacts with cathode,which makes it difficult to separate from cathode during the charging process and easier to be decomposed,thus enhancing the reversibility of Li-O2 batteries.As a result,the NiCo LDH/MnO2 bifunctional catalysts significantly reduced the discharge/charge overpotential of Li-O2 battery(the overpotential of 0.63 V at a limiting capacity of 500 mAh g-1)and provided a larger discharge capacity(the discharged capacity of 13380 mAh g-1 at a current density of 100 mA g-1).3.Using two-dimensional leaf-like CoZn-ZIF as a precursor,the Co-N bond in CoZn-ZIF was selectively broken due to the redox ability differences of different metal clusters in the bimetallic MOFs,and then in situ converted to Co(OH)2 grown on the ZIF surface,thereby obtaining a highly efficient Co(OH)2/ZIF catalyst.The stable Zn-N bonds maintain the ZIF skeleton,enabling the coexistence of micropores and mesopores.The large specific surface area and abundant pores facilitate the rapid transport of Li+and O2,providing more accessible active sites for the formation and decomposition of Li2O2.The selective disruption of the ZIF structure leads to the unsaturated Zn/Co metal centers,which generates unoccupied 3d orbitals at the metal sites,providing more catalytic sites for the reaction.Combined with theoretical calculations,the in-situ formed Co(OH)2/ZIF heterostructure can optimize the adsorption energy of reaction intermediates,lower the reaction energy barrier,and further enhance the catalytic activity for the Li-O2 battery reaction.Thanks to the efficient synergistic effect of Co(OH)2 and ZIF with unsaturated metal sites,the performance of Co(OH)2/ZIF lithiumoxygen batteries is significantly improved,exhibiting a high discharge specific capacity of 13570 mAh g-1 and a long cycle life of 275 cycles at a high current density of 500 mA g-1.4.Based on the ligand engineering strategy,CoBDC-Fc with high catalytic activity was prepared by introducing ferrocenecarboxylic acid to partially replace the original ligand in CoBDC.In view of the rapid degradation of redox mediators in Li-O2 batteries and the low conductivity and catalytic activity of MOFs,ferrocenecarboxylic acid partially replaces the original ligand and has a strong link with the metal node.The catalytic activity of CoBDC is dramatically increased after anchoring ferrocenecarboxylic acid,which accelerated the charge transfer and generated unsaturated metal sites.For ferrocenecarboxylic acid,the anchoring of CoBDC can inhibit the "shuttle effect" of redox mediators,reduce the degradation due to the attack of 1O2 and improve its stability.The LiO2 adsorption energy of CoBDC-Fc is much higher than that of CoBDC,which induces Li2O2 to form a more easily decomposable morphology through transforming from vertically growing discs to small-sized Li2O2 particles.The CoBDCFc cathode exhibits a low overpotential(1.02 V),a high specific discharge capacity(6759 mAh g-l),and a long cycle life(210 cycles)at the current density of 500 mA g-1. |
